Polycrystalline oxide bodies



y 6, 1969 A. I. BERGHEZAN 3,442,667

POLYCRYSTALLINE OXIDE BODIES Filed NOV. 29, 1963 Sheet 2 of 2 X 150INVENTOR. 3 AUREL I. BERGHEZAN gwiqwmm ATTORNEY United States Patent3,442,667 POLYCRYSTALLINE OXIDE BODIES Aurel I. Berghezan, Brussels,Belgium, assignor to UllIOll Carbide Corporation, a corporation of NewYork Filed Nov. 29, 1963, Ser. No. 327,139 The portion of the term ofthe patent subsequent to Dec. 27, 1983, has been disclaimed Int. Cl.C04b 35/10, 35/14 US. Cl. 106--39 2 Claims ABSTRACT OF THE DISCLOSURETwo polycrystalline oxidebodies formed from respective ternary melts ofAl O SiO CaO and Al O SiO CuO, the grains being ternary solid solutionscontaining at least 90% A1 0 and 90% CuO, respectively, and at the grainboundaries a thin bonding layer which is the lower melting reactionproduct of the oxides in the ternary melt.

and generally are quite susceptible to thermal shock failure. Otherapplications than these high temperature uses exist for which oxidecompositions would be attractive but the physical failings of oxidesystems also prevent their use here.

. The possibility of combining the good high temperature properties ofceramic materials with the properties of metals has been advanced manytimes and has been rather thoroughly explored in the hope of producing amaterial superior to either the ceramic or the metal alone.Unfortunately, although numerous combinations of metals and oxides havebeen tried, the strengths of the resulting materials are low withrespect to pure sintered oxide or pure metal. If the ceramic materialpredominates, the strength approximates but is below that'of the pureceramic, and similarly, if the metal phase predominates, the strength isusually substantially below that of the pure metal.

'. Reasons for the disappointing characteristics of combinations ofmetals and ceramics have been sought, and many efforts to solve theproblem have been made. It has been recognized for example that bondingbetween the metal phase and the ceramic phase is weak, but how toovercome this weakness has proved to be an extremely difficult problem.

With respect to the ceramic phase itself, it has been recognized that amaterial formed by pressing and sintering a finely divided ceramic isnot nearly so strong as a single crystal of the same material. Attemptsto increase the strength of such compacts by densification methods knownto the art are not entirely successful, for such methods lead to graingrowth, and the production of large grains tends to increase theanisotropy of the material, particularly with respect to its coefficientof thermal expansion. On the other hand, production of extremelyfine-grained material although desirable in some respect, for instancein minimizing anisotropy, does not solve the problem of strength, for itincreases the grain boundary areas in the material, and the grainboundaries are inherently weak.

Patented May. 6, 1969 It is believed that weakness at grain boundariesin polycrystalline ceramic materials can be attributed to the nature ofthe bond between atoms of the materials. This bond must be betweenneighbor atoms, and the shorter the bond, the stronger. At grainboundaries, partly because of differences in orientation of neighboringgrains, the distances between neighbor atoms are increased, and the bondis weakened.

The present invention has for its principal object an effective solutionto the problems of weakness in polycrystalline oxide compositions andweakness in the bond between grains of ceramic materials. Morespecifically, it is an object of the invention to providepolycrystalline oxide bodies having a controlled microstructureproviding strong bonds between the grains of the material whilesuppressing the formation of normal grain boundaries.

The invention will be described with reference to the accompanyingdrawing, in which:

FIG. 1 is a ternary diagram illustrating compositions embodying theinvention; and

FIGURES 2 and 3 are photomicrographs of specific materials embodying theinvention.

The invention comprises a body of selected polycrystal line oxides thegrains of which are bonded by the reaction product of the oxide and theliquid phase of a composition at least ternary in nature of the selectedoxides, the bonding compositions being distributed throughout the bodyin sites normally corresponding to grain boundaries. The oxide of whichthe body is formed may be one or more of the following: alumina; silica,which may be replaced in whole or in part by germania; and oxides ofcopper, cobalt, silver, manganese, nickel, iron, and zinc. Hereinafter,in the drawing, and in the appended claims, the designation MeO will beused to refer to this group of metal oxides for the sake of concisenessit being understood that the term is intended to include any oxide ofthe metals enumerated either alone or in combination. The bondingcomposition must have a melting point lower than the melting point ofthe selected oxide system for the body for reasons which will bediscussed below.

A fundamental concept of the invention which is believed to be broadlyapplicable is that the formation of normal grain boundaries inpolycrystalline oxide bodies must be suppressed if strength approachingthe strength of single crystal oxide is to be attained. In place ofthese normal grain boundaries, the invention provides a thin layer ofbonding composition or cement. This layer of bonding composition must bethin enough to prevent weakness normally encountered at grainboundaries. .While it is diflicult to give an exact measure for desiredthickness of the layer, it should preferably be of submicron size.Examination of the photomicrographs of FIGURES 2 and 3 will showdesirably thin layers.

Another fundamental concept of the invention is that the bondingcompositions have a melting point below that of the selected oxidesystem. By providing a liquid phase in contact with the oxide grains,excessive grain growth is prevented during formation of the desiredbody. Prevention of grain growth minimizes anisotropy and its attendantproblems. The liquid phase may react with the oxide system, anddiffusion effects may also take place, so that the composition of thethin layer in the final body may be different from that of the liquidphase present during formation of the body.

As may be seen by reference to FIGURE 1 of the drawing, the invention isconcerned with materials within small areas at the corners of theternary diagram there represented. The composition of a polycrystallineoxide body embodying the invention may comprise a substantially pureoxide as indicated or may comprise a ternary material falling within anyof the hatched areas. In any 3 event, the liquid phase of the bondingcomposition falls within any of the hatched areas but must be differentfrom, and have a lower melting point than the selected oxides for themain body.

In terms of numerical limits, the bonding composition should not exceedabout 10% by Weight of a body embodying the invention and preferably issubstantially less than 10%. Generally, one of the components of thebonding composition should be present in an amount less than 2%. Thesmallest quantity effective to form a thin layer about the grains ofoxides is desired.

The following examples illustrate the preparation of bodies embodyingthe invention.

Example I Finely divided alumina containing a small but significantproportion of silica was mixed with powdered cobalt oxide (C 0 Themixture was heated to the melting point of Al O until the formation of aliquid phase was observed. The mixture was then allowed to cool, theliquid phase of course solidifying. A dense, strong, transparent bodywas obtained.

The microstructure of this body is illustrated by the photomicrograph ofFIGURE 2 of the drawing. It will be observed that quite uniformly sizedgrains of one phase are present, these grains being outlined by a thinnetwork of bonding composition. Tests have indicated the grains to becomposed of a high alumina solid solution containing small quantities ofcobalt oxide and silica, while the bonding layer is composedsubstantially of cobalt. The composition of the body is indicated at Ain FIGURE 1 of the drawing.

Example II Powdered copper oxide (CuO) was mixed with powdered silicaand alumina in such proportions that silica constituted about 2% of themixture, and alumina was present in a proportion of about 0.5%. Themixture was heated to about 1200 C. to 1300 C. to melt all thecomponents. Upon cooling, solidification occurred, the binder phasesolidifying last. The body thus produced was strong and dense. Itsmicrostructure is typified by the photomicrograph of FIGURE 3 of thedrawing. It will be seen that uniformly sized grains are surrounded bythin layers of another material. Examination of the body has shown thegrains to be a mixture of copper oxides and the thin lays to be aternary copper oxide alumina-silica composition. The general compositionof the body is indicated at B in FIG. 1 of the drawing. This materialhas properties recommending its use for rectifiers.

It will be noted from the examples described that polycrystalline oxidebodies having the desired microstructure may be produced in accordancewith the invention by preparing a mixture of the desired oxideconstituent; providing in the mixture suflicient oxides to produce onheating a ternary (or more complex) liquid phase which wets and bonds tothe oxide grains in the mixture; heating the mixture under conditionsproducing such liquid phase; and cooling the mixture to causesolidification of the liquid material. By employing a minimum of bondingcomposition, the nearly ideal microstructure shown in the drawing isobtained.

Although the invention is applicable to the preparation ofpolycrystalline oxide bodies of widely different compositions andproperties, a common characteristic is the microstructure which providesmore nearly than has heretofore been attained optimum strengthcharacteristics. It will be apparent for example that the high aluminacontent material illustrated by FIG. 2 has excellent high temperatureproperties in view of the refractory nature of alumina, but that thematerial of FIG. 3 is not suited for use at temperatures much above 1000C. by reason of the low melting points of the phases present.Nevertheless both materials possess requisite strength for use in theirrespective fields.

It will be evident from the foregoing that the invention is capable ofwide application to a great number of materials and that the examplesgiven are illustrative of its principles rather than its scope.Substitution of other oxides for those mentioned may be made. Thus foralumina, other refractory oxides including titania, zirconia, and thoriamay be substituted in whole or in part.

What is claimed is:

1. As an article of manufacture a polycrystalline oxide body composed ofgrains of a high alumina solid solution containing small quantities ofsilica and cobalt oxide and lying within the hatched area of FIG. 1 ofthe drawing containing the point A, said grains being bonded together bya thin layer of the reaction product formed in the liquid phase of aternary composition of alumina, silica and cobalt oxide, said reactionproduct having a melting point below that of alumina and beingsubstantially all cobalt and being distributed throughout said body insites normally corresponding to grain boundaries, said cobaltconstituting less than 2% by weight of said body.

2. As an article of manufacture a polycrystalline oxide body having acomposition within the hatched area of FIG. 1 of the drawing containingthe point B and being composed of grains of copper oxide bonded togetherby a thin layer of the reaction product formed in the liquid phase of aternary composition of alumina, silica and copper oxide, said reactionproduct having a melting point below that of the copper oxide grains andbeing a ternary composition of copper oxide, alumina and silica andbeing distributed throughout said body in sites normally correspondingto grain boundaries and constituting not more than about 10% by weightof said body.

References Cited UNITED STATES PATENTS 3,282,711 11/1966 Lin 106-393,294,496 12/1966 Berghezan 106-65 2,544,060 3/1951 Amberg et al 106-653,244,539 4/1966 Hare 106-65 OTHER REFERENCES Searle: RefractoryMaterials, 1950, 3rd ed., pub. London, Charles Grifiin & Co., pp.184-185.

Gaertner: A means of Producing Copper-Copper Oxide Rectifiers, CeramicBulletin vol. 30 (1951) (pp. 265266).

Cahoon et a1., Sintering & Grain Growth of Alpha- Alumina, J. Am. Cer.Soc. vol. 39 (1956) (pp. 337-344).

HELEN M. MCCARTHY, Primary Examiner.

U.S. Cl. X.R.

